This invention relates to an incandescent lamp at is turned on with a coil axis of a filament being directed in a vertical direction, and, for e ample, an incandescent lamp filled with inert gas containing halogen.
In the case of a usual incandescent lamp of the one-end sealed type which is widely used, for example, a coil-like filament within a cylindrical glass bulb is connected to upholding parts also acting as an inner lead wire at both ends of the filament, and the filament is held and arranged in such a way that a bulb axis of the bulb and an axis of the coil are coincident with each other.
In the case of such an incandescent lamp as described above, the lamp is turned on while it is kept in a vertical orientation. In doing so, it may sometimes happen that, when the time of the turning-on operation expires, the coil of the filament is extended and hangs out.
In particular, in the case of an incandescent lamp with the coil axis of the filament being oriented in a vertical direction as described above, a load corresponding to the weight of the coil itself is applied to the filament, resulting in that the stress of the net weight of the filament increases from the lower end of the filament to its upper end and the coil hangs out considerably at the upper end of the filament.
Since both upper and lower ends of the filament of the incandescent lamp as described above are held and fixed by the upholding parts, the filament length and the number of turns of the coil however do not change, and when the coil hanging-down state is generated above the filament the pitch of the coil positioned further below is gradually narrowed and the coils which are present adjacent to each other in a vertical direction get in contact with each other. As a result, a short-circuit state or a melting cut of the filament occurred and turned off the lamp resulting in the problem of making the lamp life short.
In particular, in the case of an incandescent lamp in which the turning-on or turning-off operation was repeated within a short period of time under such a high temperature that the temperature of the filament exceeded 2600xc2x0 C., for example, and the lamp was turned on in a periodic manner, the hang-down of the coil was produced remarkably often and caused the lifetime of the lamp to become quite short.
In view of the aforesaid problem, it is also possible to fix the upholding parts not only to the upper and the lower ends of the filament, for example, but also over a longitudinal direction of the filament and to prevent the coils from getting in contact with each other. However, application of such a prior art method as described above causes the structure of the filament to become a complicated structure and further its productivity to become quite poor.
In addition, in the case that the incandescent lamp is utilized as a heat source, in particular in the case that the filament of the incandescent lamp has a size relation wherein the length of the filament and the diameter of the coil are identical with each other, this may cause the thermal efficiency of the lamp to be reduced, resulting in that it is preferable that the number of upholding parts is as low as possible.
In view of the foregoing, it is an object of the present invention to provide an incandescent lamp in which an unlit state as well as a short lifetime of the lamp and a hang-out of the coil is prevented by a simple configuration and its lifetime in practical application is long.
In view of the foregoing, in a first aspect of the present patent application there is provided an incandescent lamp in which a coil-like filament is stored in a bulb and the filament is held by upholding parts with an axis of the coil being oriented in a vertical direction, wherein the coil is made such that a coil pitch at the upper part of the filament is made narrower than a coil pitch at the lower part of the filament.
With such an arrangement as above, when the coil hangs out considerably as described above at the upper part of a usual filament and the coil extends to hang down, the pitch of the coil is made narrow at the lower side of the filament resulting in that the filament may contact the coil. In accordance with the first invention, the upper coil is wound more closely than the lower coil of the filament without changing the filament length or the number of turns of the coil, the coil pitch of the upper coil of the filament is made narrower in advance than that of the lower coil in anticipation of a deformation of the coil, and the pitch of the lower coil of the filament is wound wide. In doing so, even if the coil hangs out to cause the pitch of the lower coil to be narrowed, it requires a lot of time until the coils themselves get in contact with each other, so that the contact of the coils can be avoided and, further, it becomes possible to prevent a short-circuit state and a melting cut of the filament. Then, as a result, it is possible to prevent an unlit state of the lamp and attain a long lifetime of the lamp.
Further, in a second aspect of the present patent application there is provided an incandescent lamp in which a coil-like filament is stored in a bulb and the filament is held by the upholding parts with an axis of the coil being arranged in a vertical direction and wherein the coil has a conical shape or its shape is a frustum of a circular cone and it is arranged with its one end having a smaller coil diameter being placed to the upside. With such an arrangement as above, since the shearing force applied to the coil is reduced relative to the axial load of the coil as the coil diameter is decreased, resistance against the stress applied to the coil, i.e. the anti-hang out characteristic, is increased. Accordingly, even if the load is increased from the lower end of the filament to the upper end thereof, a gradual reduction of the coil diameter causes the anti-hang out characteristic of the coil to be increased and, accordingly, the hang-out of the coil can be effectively restricted. Thus, deformation caused by the hang-out of the coil is hardly produced even at the upper part of the filament so that it is possible to prevent the coil pitch below the filament from being narrowed, and it is also possible to avoid contact between the coils themselves.
As a result, it is possible to prevent a short-circuit state and melt-cut of the filament, to prevent an unlit state of the lamp and to attain a long lifetime of the lamp. Even if a remarkable hang-out of the coil is produced, it is possible to extend the lifetime of the lamp.